In a normal Ni-yttria stabilized zirconia (YSZ) anode of a solid oxide fuel cell (SOFC), which is operated on externally reformed methane, there is a temperature distribution over the cell in the order of about 150° C. at an operational temperature of 850° C. Such a gradient has a detrimental effect on the mechanical as well as the chemical durability of the cell, and can for instance cause mechanical failure or enhanced chemical reactions in the warmest regions, as discussed, for example, in N. Q. Minh, and T. Takahashi, Science and Technology of Ceramic Fuel Cells, (Elsevier Science B. V., Amsterdam NL, 1995), and High Temperature Solid Oxide Fuel Cells: Fundamentals, Design and Applications, Eds. S. C. Singhal and K. Kendall. This will in return inevitably result in a performance decrease of the cell over time. It is therefore desired to level the temperature gradient, as this will result in an overall increase in cell performance because the temperature of the colder parts of the cell is increased.
In the case of an internal reformation of wet natural gas, such as methane, in the cell, the temperature gradient will be even steeper than described above due to the endothermic reforming process at the inlet, and will thus be more damaging to the cell (and stack), as disclosed, for example, in Hendriksen, P. V., Model studies of internal steam reforming in SOFC stacks, Proceedings—Electrochemical Society (1997), 97-40 (Solid Oxide Fuel Cells V).
However, if dry natural gas is employed instead, the formation of carbon will result in a fast blocking of the active sites in the anode structure. This may be prevented by using, e.g. an all-ceramic anode for direct conversion of natural gas to CO, CO2 and water at the inlet which further down the stream may be replaced by Ni-containing electrode for a more efficient conversion. Carbon formation may also be prevented by a slower conversion at the inlet.
It has been suggested to use electrodes for SOFC or other electrochemical devices which are graded vertically to optimize the ionic and electronic conductivity of the electrodes. U.S. Pat. No. 5,543,239 discloses an improved electrode design for solid state devices, wherein a porous layer of electrolyte material is incorporated over the dense electrolyte, and wherein an electrocatalyst which is also continuous is incorporated into the porous layer.
EP-A-1701402 relates to systems and methods for minimizing temperature differences and gradients in solid oxide fuel cells. A manifold heat exchanger is used, which reduces thermal stress and increases cell life. Air passes from a periphery of a cell towards the cell centre, absorbs the heat, and proceeds to the manifold heat exchanger adjacent to the cell, where it absorbs further heat. Fuel is directed counter current to the air, which keeps hot spots away from the cell stack seals and directs hot air towards intense reforming areas on the cell to mitigate quenching effects of internal reforming.
U.S. Pat. No. 6,228,521 concerns a high power density solid oxide fuel cell having a cathode, electrolyte and graded porous anode. The anode is formed from NiO and zirconium oxide doped with yttrium oxide and exhibits a graded density which allows thicker and thus stronger anodes without sacrificing electrochemical performance.
U.S. Pat. No. 4,329,403 relates to an electrolyte-electrode assembly for high temperature fuel cells in which the electrolyte member is adapted to exhibit a more gradual transition in coefficient of thermal expansion in going from the anode electrode to the inner electrolyte region and in going from the cathode electrode to the inner electrolyte region.
U.S. Pat. No. 5,171,645 discloses a graded metal oxide electrolyte comprising gradiations of zirconia and bismuth oxide across the cross-section of the electrolyte. The gradiation of the compositional content across the wall thickness of the electrolyte from a substantially pure zirconia surface zone to a substantially pure bismuth oxide-yttria surface zone minimizes the stress at the interfaces between the various compositional zones.
US-A1-2005/0092597 relates to a method of forming a thin-film fuel cell electrode, comprising the provision of a substrate and at least one deposition device; developing a deposition characteristic profile having at least one porous layer based on pre-determined desired electrode properties; and forming a film in accordance with said deposition material from said deposition device while varying a relative position of said substrate in relation to said deposition device with respect to at least a first axis.
As the deposition device, sputter guns are used. However, said sputter guns result in varying thickness as the overlapping areas are thicker than the surrounding areas, which leads to unwanted variations of the layer properties in a horizontal direction. Furthermore, with sputtering, only thin layers well below 1 μm can be provided, which is too thin for applications in solid oxide cells as employed today, where well performing electrodes typically has electrochemical activity in thicknesses of 10-15 microns. Moreover, sputtering is a very slow and expensive process and thus is very cost intensive, prohibiting large scale production. Further to that it is difficult to achieve porous layers and there is also a significant waste material when sputtering methods are used (preparation of targets and material deposited outside of the substrate).
J. A. Labrincha et al., “Evaluation of deposition techniques of cathode materials for solid oxide fuel cells”, Mat. Res. Bull., Vol. 28, pp. 101-109, 1993, discusses specific cobaltates and manganates as cathode materials for solid oxide fuel cells, wherein the cathode layer was applied on an electrolyte layer by sputter deposition.
EP-A-1441406 discloses a method for making a fuel cell anode, comprising the steps of:                depositing a first film on a first end region of a substrate, wherein the first film is preferentially catalytically active towards substantially unreformed hydrocarbon fuel; and        depositing a second film on a second end region of the substrate, the second end region opposed to the first end region, wherein the second film is preferentially catalytically active towards at least one of substantially reformed or partially reformed hydrocarbon fuel, by-products thereof and mixtures thereof.        
US-A-2004/0086633 discloses a method for the fabrication and evaluation of electrode and electrolyte materials for use in solid oxide fuel cells, the method comprising:                providing a non-sintered or partially-sintered substrate; and        delivering the electrode and electrolyte materials to a plurality of regions of the substrate using a plurality of liquid spraying devices, wherein the plurality of liquid spraying devices are arranged at appropriate angels to the substrate and to each other such that the spray plumes of the spraying devices overlap to form a gradient array.        
Z. Wang et al., “A study of multilayer tape casting method for anode-supported planar type solid oxide fuel cells”, Journal of Alloys and Compounds 437 (2007) 264-268, relates to a multilayer tape casting and co-sintering process to fabricate a large area anode-supported electrolyte film for reduced temperature solid oxide fuel cells.
However, there is still a need for graded multilayer structures having a suitable thickness of ca. 10 microns or more, which may be used as anodes in solid oxide fuel cells, which have a sufficient horizontal grading without the layer having any significant thickness variations, having an improved life time, and which can be produced in a cost efficient way without much waste material in view of the desires and requirements for industrial large scale production.